In a continuously variable transmission, it has been well known to use friction between a metal belt or chain and a pulley made of metal to transmit torque. A lubricating oil used for such a continuously variable transmission is required to have a high power (torque) transmission capacity. Accordingly, there have been known various kinds of lubricating oil compositions with a large power transmission capacity enough to allow a favorable power transmission (e.g., see Patent Documents 1 and 2).
A lubricating oil composition disclosed in Patent Document 1 is excellent in abrasion resistance and extreme pressure property and is capable of maintaining a high friction coefficient for a long time. In order to transmit a large amount of torque, this lubricating oil composition is formed by adding a base oil of lubricating oil with a sulfuric extreme pressure agent, a phosphorus extreme pressure agent and an alkali earth metal detergent.
A lubricating oil composition disclosed in Patent Document 2 is formed by adding a base oil of lubricating oil with an effective amount of succinic bisimide having a predetermined structure in order to increase the power transmission capacity of the lubricating oil composition and to improve the anti-shudder property thereof.
In a lubricating oil composition disclosed in Patent Document 3, a sulfonate detergent and boron-containing succinimide are mixed, thereby improving the torque transmission capacity of the lubricating oil composition and the antiwear property of a wet-type friction agent.
[Patent Document 1] JP-A-9-100487
[Patent Document 2] JP-A-9-202890
[Patent Document 3] JP-A-2007-126543